The present application relates to a recording medium including a recording layer in which interference fringes formed in parallel with the surface of the recording medium are deleted or changed within a portion irradiated with focused light to record information or light reflected during irradiation with focused light is used to reproduce information, and a reproducing apparatus for the recording medium.
In optical disc systems such as a CD (compact disc), DVD (digital versatile disc), and BD (blu-ray disc®), minute changes in the reflection coefficient made on one side of a disc are read in a noncontact manner just like the objective lens of a microscope. The size of a light spot on a disc is obtained by λ/NA (λ: wavelength of irradiating light where NA represents a numerical aperture), and the resolution is proportional to this value. For example, a blu-ray disc with a diameter of 12 centimeters has a capacity of approximately 25 gigabytes.
In addition, overlapping a plurality of recording layers increases the capacity of one disc.
On the other hand, a method of recording stationary waves has been proposed.
After light is focused on an inside of a recording medium such as an optical disc whose refraction index depends on the strength of irradiating light, light is focused on the same focal point again from the opposite direction by a reflection apparatus disposed on the backside of the recording medium. This forms a hologram with a small light spot to record information.
During reproduction, information is identified by reading the reflected light of irradiating light from the disc surface similarly. In addition, multi-layer recording is enabled by recording information in a plurality of layers in an optical recording medium.
In this method, however, an optical system should be disposed on each of the upper and lower sides of a recording medium such as an optical disc, so the entire optical system or drive system becomes large and complicated.
A method that records (preformats) interference fringes the entire surface in an optical disc and deletes or changes parts of the interference fringes to record marks is proposed in “Three-dimensional optical disk data storage via the localized alteration of a format hologram”, by R. R. Mcleod, A. J. Daiber, T. Honda, M. E. McDonald, T. L. Robertson, T. Slagle, S. L. Sochava, and L. Hesselink, Appl. Opt., Vol. 47 (2008) pp 2696-2707.
A method that records marks by changing the refraction index of the material of a recording layer without creating interference fringes is proposed in “Fabrication of multilayered photochromic memory media using pressure-sensitive adhesives”, by Y. Kawata, et. al., Appl. Opt., Vol. 46 (2006) pp 8424.